Product description sigmarsquos 17 50mm f28 ex dc os hsm large aperture standard zoom lens covers a focal length from 17mm wide angle and offers a large aperture of f28 throughout the entire zoom range making it ideal for many types of photography especially portraiture and landscapes the os sigmarsquos anti shake feature offers the use of shutter speeds approximately 4 stops slower than would otherwise be possible the os feature allows sony and pentax users the option of viewing a stabilize